Capacitive micromachined ultrasound transducers (CMUT) have emerged as a viable alternative to traditional piezoelectric transducers. In general, a CMUT is essentially a micron-sized air-gap or vacuum-gap capacitor that, by electrostatic effects, can be used for the generation and detection of acoustic/ultrasonic waves. Applications of CMUT arrays include medical ultrasonic imaging and underwater imaging, as well as air applications such as nondestructive evaluation (NDE) and nondestructive testing (NDT).
Conventionally, a CMUT array is usually fabricated on a front side of a silicon substrate using surface micromachining technologies. For ease of fabrication and access to the individual CMUT cells, a control electrode for accessing each CMUT cell is also formed on the front side of the silicon substrate. This arrangement makes inefficient use of the surface area on the front side of the silicon substrate, and requires long routing lines to address the CMUT cells, especially for two-dimensional CMUT arrays. The long routing lines can introduce parasitic capacitance and resistance, resulting in sub-optimal performance of the CMUT array.